This invention relates to image transfer processes and elements used therein. More particularly, the invention relates to a light sensitive element which, after imagewise exposure and removal of unexposed areas, can be utilized as a transfer element whereby individual image areas can be transferred to a receptor surface.
Dry transfer sheets consist of a transparent support carrying thereon indicia such as letters, numerals, or other symbols which can be individually transferred to a receiving surface, such as a sheet of paper. Transfer occurs by application of rubbing pressure to the rear surface of the support while the image contained on the support is in contact with the receptor, followed by peeling away the support, whereupon the image adheres to the receptor surface.
A conventional technique for manufacturing dry transfer sheets at the present time is to screen print indicia on a transparent support. Such manufacture is costly and complex, generally being undertaken only by experts in the field. For each of the numerous indicia which might be required in practice, a separate screen stencil is necessary, thereby economically limiting the manufacture to larger quantities of indicia having the greatest utility in the field. The cost of providing small numbers of special-purpose indicia for individual uses is therefore prohibitive.
Recently, transfer elements utilizing light sensitive systems have been disclosed. In U.S. Pat. No. 3,671,236 a method for photo mechanically producing multicolored images on a single substrate is disclosed. In that disclosure, the entire light sensitive diazo resin-based layer is transferred from a carrier sheet to a receptor prior to imagewise exposure thereof. Imaging and development is undertaken subsequent to transfer. There is no disclosure of transferring individual indicia comprising only the light exposed areas of the transfer element. Additionally, the light sensitive system therein is based on a diazo resin as opposed to free radical photopolymerizable materials.
Free radical photopolymerizable materials have been utilized in image transfer, generally based on the fact that a differential degree of surface tack exists between unexposed and exposed areas of a photopolymer system. Unexposed areas of the photopolymer layer generally have a greater degree of tack than exposed areas, thereby affording ability to selectively transfer unexposed areas to a receptor. In U.S. Pat. No. 3,342,593 this tack differential occurs upon heating an imagewise exposed transfer element, while in U.S. Pat. No. 3,202,508, tack differential occurs in situ upon light exposure.
In U.S. Pat. No. 3,525,615, a thixotropic gel is interspersed in the photopolymerizable layer. After imagewise exposure, unexposed areas are capable of liquifying and transferring when contacted with a receptor surface under high pressure.
In all of these transfer processes utilizing photopolymerizable materials, visual inspection of the transferrable image areas is generally unavailable because the exposed and unexposed areas of the light sensitive layer are indistinguishable and remain as an integral layer until the moment of transfer. Additionally, selective transfer of individual indicia is generally unavailable. Furthermore, after light exposure, the transferrable image portions remain light sensitive, thereby necessitating storage and transfer under subdued light.
It is of course known that a light exposed photopolymerizable stratum can be imagewise developed with a developing medium to thereby remove unexposed areas of the stratum. See Light Sensitive Systems: Chemistry and Application of Nonsilver Halide Photographic Processes, J. Kosar, J. Wiley and Sons (New York, 1965). To our knowledge, however, it has never been disclosed that the exposed polymerized image areas remaining after development can be transferred to a receptor.
British Pat. No. 1,336,065 discloses a light sensitive transfer element which can be visually inspected prior to transfer. A component capable of forming a gas upon light exposure is utilized in conjunction with a gas impervious binder resin in the transfer layer. The gas causes the bond between the transfer layer and the support to be weakened, thereby allowing removal of the exposed portions from the support with a developing medium, the unexposed portions remaining. These remaining unexposed portions are then light exposed a second time to again weaken the bond between the support and image areas and thereby provide indicia which are transferrable to a receptor element by pressure. However, this transfer system provides images with limited resolution, since the binder in the transfer layer must be able to retain the gas formed upon exposure, thereby limiting the sharpness of image boundaries. Similarly, control of the release of the transfer layer from the support is difficult to maintain. Additionally, this transfer element must be light exposed twice before transfer can be effected.
In contrast to the aforementioned transfer systems, my invention comprises a light-sensitive transfer element which, upon a single imagewise exposure and development, provides pressure transferrable indicia having excellent resolution which can be visually inspected prior to tranfer. Since the light exposed portions are transferrable, indicia transfer can be undertaken at normal room temperature and light conditions utilizing a simple stylus.